Both common forms of motor used in test apparatus, servo-hydraulic and electromagnetic, comprise output shafts with large masses. Should a fault occur within the apparatus, the large mass can accelerate and cause damage to both the apparatus and any persons in the vicinity. Braking systems are therefore designed to prevent undesired movements of the output shaft.
Servo-hydraulic systems may comprise mechanical brakes whereby the movement of the output shaft is managed by gripping or releasing the output shaft and/or the movement of the output shaft can be managed by controlling the flow of oil in the hydraulic cylinder.
Electromagnetic motors are not operated by the same principles as a servo-hydraulic motor. Instead, an output shaft comprising magnetic material is positioned within a coil assembly, alternatively, moving coil electromagnetic motors mount the coil assembly to the output shaft and positioned within a housing comprising magnetic material. The coil assembly comprises a plurality of separate coil loops, an electric current signal applied through the coil assembly induces a magnetic field which interacts with the magnetic material's magnetic field and therefore a force on the output shaft is generated dependent on the magnitude and direction of the applied current. This force can be used to accelerate or decelerate a moving output shaft, additionally it can be used to counteract gravitational force acting on the output shaft when the test apparatus is in a vertical orientation or any orientation other than horizontal. In a non-horizontal orientation, it will be appreciated that in the absence of electrical power, the output shaft is free to move under the effect of gravity. Therefore, a mechanical brake must be actuated to prevent undesired movement of the output shaft such as when the motor is switched off. An additional means for braking a moving output shaft of an electromagnetic motor is through electromagnetic induction whereby motion of the output shaft within the coil assembly induces eddy currents in the output shaft so as to generate an electromagnetic braking force between the output shaft and the coils otherwise known as an e-brake.
While there is no current standard for compliance which dictates how quickly an output shaft with undesired motion must be arrested, it is desirable that motion be arrested prior to the output shaft travelling 2 mm or less and/or in the case of rotary motors: 360 degrees. Due to the large combined mass of the output shaft, it is possible for the output shaft to travel the length of its travel within a few milliseconds when acting only under gravity. Thus, the response time of the braking system in engaging either the mechanical brake, or reversing the current applied to the coil assembly or applying an e-brake is required to be very short to prevent 2 mm or of travel.
Several known methods for controlling the actuation of the different braking methods involve monitoring the velocity of the output shaft. In an ideal situation, a perfect Safely Limited Speed (SLS) mechanism is used to pre-emptively prevent the output shaft from achieving speeds greater than a predetermined threshold—typically 10 mm/s or 30 deg/s for rotary movement. Unfortunately, current SLS mechanisms are unable to perform this desired function; they operate by monitoring the velocity and raising a fault condition when the threshold is exceeded. Therefore, the velocity of the output shaft has already exceeded the threshold once the fault condition has been raised—the SLS does not pre-emptively limit the speed to avoid exceeding the threshold, it reacts once the threshold is exceeded. This is equivalent to a Safe Speed Monitor (SSM) that monitors velocity and generates a safety and/or fault signal when the velocity is below or above a predetermined threshold, in conjunction with a Safe Torque Off (STO) mechanism that shorts the windings on the coil assembly producing a magnetic braking effect. Similarly, monitoring of the velocity can be extended to monitoring of the acceleration of the output shaft wherein the STO is engaged when a predetermined acceleration threshold is exceeded—typically 30 mm/s2 or 90 deg/s2 for rotary movement.
Several problems exist in relation to the above braking system that uses an SSM in conjunction with an STO to prevent undesired movement. There are scenarios that exist in which the threshold for either velocity or acceleration can be exceeded but a fault has not occurred. These are termed false-triggering events or nuisance tripping. In these scenarios, the braking system engages and the apparatus is made safe until an inspection can be carried out. The frequency at which these false-triggering events occur results in a sizable proportion of operators for test apparatus switching off the emergency braking system; a practice which can lead not only to apparatus damage but also injury to operators.
Examples of scenarios where the threshold for either velocity or acceleration is exceeded but a fault has not occurred include (but are not limited to):
1. Each time the mechanical brake is released, an impulse is experienced by the output shaft resulting in large acceleration and velocity.
2. Working in the environment with the test apparatus, the apparatus can be knocked accidentally, resulting in large acceleration and velocity.
3. Adjusting the load string with tools can result in impulses that produce large acceleration and velocity.
4. Operating the specimen holding grips can result in impulses that produce large acceleration and velocity.
5. When operating in a set-up mode, in which the current applied to the motor is strictly limited, ‘motor cogging’ can result in the velocity and acceleration thresholds being exceeded.
There is therefore a need to develop an emergency system that has a high accuracy in differentiating false-triggering events from actually undesired output shaft motion.
Conventional means for arresting the motion of an uncontrolled output shaft is the use of an electrical brake: by connecting each of the plurality of the separate coil loops of the coil assembly to each other. To do this, a mechanical device is used; a delay between the instruction to apply the e-brake and the mechanical device enacting the brake results in the time between identifying a failure event and the output shaft being arrested being of the order 10 milliseconds. A delay of this duration is not sufficient to bring the output shaft to rest within 2 mm of travel and as such a new device is required to decrease the distance of travel by reducing the delay time.
Test apparatus must abide by certain safety standards, one such standard involves the lifetime of the mechanical brake pads used in each mechanical brake. It is therefore desired to produce a means for monitoring the lifetime of brake pads fitted to a mechanical brake.